In the formation by extrusion of tubular bodies, e.g. large-caliber pipe, from a thermoplastic synthetic-resin material, the synthetic-resin material is transformed into a thermoplastified state in a masticating and plastifying unit operating with one or more worms and forcing the material continuously out of the discharge end of this unit. The unit is generally referred to as an extrusion press and, at the mouth thereof, is provided with the shaping tool or extrusion die which imparts the desired configuration to the tubular body which is to be produced.
The extrusion die, which is connected to the discharge head of the press generally comprises an outer tool or member, usually in the form of a sleeve, and an inner tool or member, usually in the form of a mandrel around which the synthetic-resin material is forced.
To establish the outer diameter of the extruded strand or tube, the outer member of the die can have connected thereto a so-called calibrating sleeve which can engage the outer surface of the extruded body and in part the desired external diameter thereto. The calibrating sleeve may be cooled so that the extruded tube is at least partially hardened by the calibrating process.
It is also known to provide such apparatus with a thickness-measuring device utilizing radioactive material or, more generally, a radioactive source disposed on one side of the pipe wall and a radiation detector disposed on the opposite side of this pipe wall.
The outer member of the die may consist, in turn, of a plurality of parts and, in general, these include a fixed outer tool part and an adjustable outer tool part, the adjustable outer tool part being shiftable relative to the mandrel or inner tool part by at least three angularly equispaced adjusting screws.
A wall thickness-measuring device of the type mentioned previously can constitute means for determining whether the tube wall is of constant thickness over the entire periphery or circumference of the tube and/or merely the thickness of the tube at a given point.
With the aid of the centering-screw arrangement, the adjustable outer tool part can be shifted relative to the inner tool so that a uniform wall thickness is maintained around the entire periphery of the extruded product.
By controlling the rate at which the synthetic-resin material is fed into the die by the extrusion press and the rate at which the extrusion tube is withdrawn from the die, the mass flow can be controlled to establish the wall thickness per se.
In conventional apparatus of the aforedescribed type, the wall-thickness measuring device is not integrated with the extrusion die and is usually not even connected thereto. It is customary to provide the thickness-measuring device as an independent unit disposed downstream of the die and even downstream of the cooling stretch along which the tube passes. The radioactive thickness detector thus operates on an already hardened tube.
Thickness measurement is effected by detecting transmitted radioactivity or by detecting back-scattered radioactivity and in the latter case the detection head is disposed externally of the extruded tube. In practice, the irradiating and back-scatter detection means thus encircle the extruded pipe.
The conventional apparatus can only be used with hardened extruding pipes because the apparatus which must encircle the same is guided upon rollers which engage the outer periphery of the tube and would distort the latter if it was insufficiently hardened or still somewhat plastic.
The prior-art system thus provides a considerable stretch between the extrusion die and the measuring device and, at high production rates, a considerable length of extruded tube can be produced between the detection of an irregularity by the thickness-measuring system and a correction at the extrusion head. Once a correction is made at the extrusion die or tool, it is not detected until the modified tube portion reaches the thickness-measuring device. Thus there is a considerable dead time in the control of the thickness of the pipe which has been found to be especially disadvantageous when large diameter tubes are produced.
As a result of this dead time, there cannot be an immediate response to the generation of control signals. The pipe which is produced may have detrimental deviation from the desired tolerances and hence relatively large dimensional variations. Finally, large segments of pipe may be unusable.
It has been found to be technically very costly and difficult to provide separate signals for an automatic centering of the tube head in response to thickness differences around the periphery of the tube and for affecting the output of the extrusion press and/or the tube withdrawal device to thereby control the overall thickness. In practice such withdrawal-speed controllers which regulate the total mass flow of the synthetic-resin material per unit time and therefore the wall thickness and which must be responsive to a corresponding setpoint value, is very expensive and does not always operate satisfactorily.